The invention relates to a method for field-oriented operation to zero speed of an encoder-less asynchronous machine, wherein the associated field-oriented regulation has a machine monitor with rotation-speed adaptation.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Operation of an asynchronous machine without a rotation-speed encoder is of major importance for many industrial applications. In addition to cost saving reasons, this results in the advantage of installation reliability in the case of relatively high power installations, since, from experience, rotation encoders actually make a not insignificant contribution to the failure probability of the installation.
Because of the system characteristics of asynchronous machines, field-oriented operation without information about the measured rotation speed, at a low feed voltage frequency, is critical. Industrial applications in which operation at low frequency down to a frequency of zero plays an important role, are for example for the starting of very high inertias adjacent to the torque limit in the paper industry, in the case of centrifuges which are loaded and unloaded at low rotation speeds, for defined prestressing of a drive, for example when building up tension on spools in the steel industry or for extruders in the plastics industry, which have to produce a defined torque virtually at zero speed.
The problem to be solved technically and on which this invention is based is field-oriented operation of asynchronous machines without a rotary encoder down to the point where they are stationary “zero frequency”, or down to the stationary zero-speed state, with the restriction that the load at low frequencies acts only reactively on the torque of the asynchronous machine. In the following text, a load such as this will be referred to as a passive load.
Solutions are therefore of particular technical interest which allow stable operation, without the use of sensors, for applications of this type.
In conventional field-oriented regulation devices, motor models use the stator variables “terminal voltage” and “motor current” of the asynchronous machine without a rotation-speed encoder to calculate the estimated information about the field orientation and the rotation speed of this asynchronous machine without the rotation-speed encoder. From the structural point of view, the motor model acts like a sensor which uses the electrical variables to provide the information about the field and the rotation speed of the asynchronous machine without a rotation-speed encoder to superordinate structures of the current regulation and rotation-speed regulation of a field-oriented regulation device.
Simple model versions using a forward structure such as the EMF model or the voltage model provide usable estimated variables up to about 10% of the motor rated frequency. More complex models, such as the machine monitor, in which a calculated model error reacts on the calculation of the estimated variables, achieve a minimum frequency, from experience, up to a factor of three times less.
The complete machine monitor with estimation of the rotation speed simulates the differential equations of the machine as realistically as possible, from the system point of view. Assuming exact model parameters, the expectation is therefore that the model structure will have the same stable rest point even at the frequency of zero. However, practical experiments have not confirmed this. Even after a short time, the model states current and flux diverge from one another in parallel but opposite directions. The model is entirely error-oriented with respect to the actual machine flux, and becomes unstable. Mapping errors of actual pulse-controlled inverters, measurement errors in the currents or possibly also in the measured voltage are effectively superimposed as error sources and are subject to a constant component at the “zero frequency” operating point. Since the control path has a bandpass characteristic at a stator frequency of zero, it is not possible to compensate for the constant component error, as a result of which the monitor is limit-stable. This means that errors from an indefinitely small constant component allow the system states to drift to the limit, inevitably, over time, with the consequence that the monitor becomes unstable.
For operation below the critical frequency, the rotation-speed estimation and therefore the rotation-speed regulation are switched off. When the frequency falls below a defined minimum, operation of the asynchronous machine without an encoder regulated on a field-oriented basis is disconnected, and a change is made from open-loop controlled operation. This means that the current magnitude and frequency are predetermined as fixed by a nominal-value source. Depending on the complexity, the change to and from this operating mode is more or less free of transience. However, this solution has the disadvantage that the drive can be operated with only restrictive dynamics at a relatively low frequency, and it is not possible to preset a defined torque.
Although the open-loop controlled method itself results in a major gain from the system engineering point of view, depending on the disconnection quality that is achieved, the serious disadvantage of loss of field orientation nevertheless remains. Furthermore, “open-loop controlled disconnection” results in considerable additional complexity.
DE 10 2007 003 874 A1 discloses a method for operation, without the use of an encoder, of a converter-fed asymmetric rotating-field machine, in particular of a permanent-magnet synchronous machine, by using a test signal. An estimated flux situation is continuously corrected by means of this test signal, which is fed into the rotating-field machine in the direction of a supposed flux axis of said rotating-field machine. In this known method, at least two alternating signals at a different frequency are preset at the same time as the test signal. These two alternating signals at different frequencies result in at least two different reactions, from which any discrepancy between an estimated flux situation and an actual flux situation of the rotating-field machine can then be determined with better accuracy.
The methods for operation, without an encoder, of a converter-fed rotating-field machine using a test signal are already in industrial use for permanent-magnet synchronous machines in which the rotor position is estimated. Unfortunately, this use is disproportionately more difficult for asynchronous machines without encoders due to the system characteristics.
Even this method with a test signal always requires an apparatus to generate a specific test signal. Furthermore, additional actions must be carried out in order to obtain information about the flux from reactions to one or more test signals.
The steady-state zero frequency point is a singular point with respect to the parameter “rotation speed”. If the machine voltage is constant over time, the map of the rotation speed onto the machine current is a bandpass signal. At this point, it is therefore impossible to determine a steady-state rotation speed by measurement of machine voltages and machine currents.
Parameter errors of the motor model, mapping errors of a real pulse-controlled inverter of a voltage DC-link converter, and, depending on the current measurement method, more or less pronounced measurement noise are superimposed on this singular operating point and lead to the estimated variables “rotation speed” and “field orientation” being subject to considerable errors. If these variables are applied as supposed actual values to the cascaded structures of current regulation and rotation-speed regulation of a field-oriented regulation device, then this leads to uncontrolled stimuli in the actual torque and, in consequence, to movement of the motor shaft of the asynchronous machine without an encoder, or even to this asynchronous machine departing from the steady-state operating point.
It would therefore be desirable and advantageous to obviate prior art shortcomings by providing an improved method for field-oriented operation to zero speed of an asynchronous machine, wherein the asynchronous machine operates without an encoder.